math/rand: avoid use of math.Pow in tests.
math/rand: avoid use of math.Pow in tests.

The use of math.Pow for mere squaring can be extremely
slow on soft-float ARM. Even on systems with hardware
floating-point, a speedup in test duration is observed.

On amd64
Before: ok      math/rand       2.009s
After:  ok      math/rand       0.340s

Fixes issue 3740.

R=dave, golang-dev, r, r
CC=golang-dev
http://codereview.appspot.com/6348061

wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old